Method for manufacturing a titanium alloy bar

ABSTRACT

A method for manufacturing an α+β titanium alloy bar comprising hot rolling an α+β titanium alloy consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and a balance of Ti, while keeping the surface temperature thereof to a temperature of β transus or below.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No. 10/418,252 filed Apr. 17, 2003, which is a continuation application of International Application PCT/JP02/01710 (not published in English) filed Feb. 26, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a titanium alloy bar having excellent ductility, fatigue characteristics and formability, particularly to an α+β type titanium alloy bar, and to a method for manufacturing thereof.

2. Description of Related arts

Owing to high strength, light weight and excellent corrosion resistance, titanium alloys are used as structural materials in the fields such as chemical plants, power generators, aircrafts and the like. Among them, an α+β type titanium alloy occupies a large percentage of use because of its high strength and relatively good formability.

Products made of titanium alloys have various shapes such as sheet, plate, bar and so on. The bar may be used as it is, or may be forged or formed in complex shapes such as a threaded fastener. Accordingly, the bar is requested to have excellent formability as well as superior ductility and fatigue characteristics.

FIG. 1 shows a typical manufacturing method of bar.

An ingot prepared by melting is forged to a billet as a base material for hot rolling. As shown in FIG. 2A and FIG. 2B, the billet is hot rolled to a bar after reheated in a reheating furnace using a reverse rolling mill or tandem rolling mills. If necessary, the billet is intermediately reheated during hot rolling to compensate the temperature needed for subsequent hot rolling.

As for a titanium alloy bar, particularly as for an α+β type titanium alloy bar, however, the temperature of billet increases during hot rolling owing to the adiabatic heat, which disturb's stable hot rolling and manufacturing of a titanium alloy bar having excellent ductility, fatigue characteristics and formability. For example, if the temperature of billet increases to β transus or above, the finally hot rolled bar has β microstructure consisting mainly of acicular α phase, thus failing in attaining superior ductility and fatigue characteristics. In addition, even as for a Ti-6Al-4V alloy having a high β transus, the increase in temperature during hot rolling owing to the adiabatic heat enhances grain growth, although the temperature during hot rolling hardly exceeds β transus, thus failing in attaining excellent ductility, fatigue characteristics and formability.

To solve the problem of temperature increase during hot rolling caused by the adiabatic heat, JP-A-59-82101, (the term “JP-A” referred herein signifies the “unexamined Japanese patent publication”), discloses a rolling method in which cross sectional area reduction rate of billet is specified to 40% or less per rolling pass in a region or in α+β region. JP-A-58-25465 discloses a method in which billet is water cooled during hot rolling to suppress the temperature rise caused by the adiabatic heat. Furthermore, Article 1 “Hot Bar Rolling of Ti-6Al-4V in a Continuous Mill (Titanium '92 Science and Technology)” describes that hot rolling speed is reduced to the lower limit of keeping performance of mill in order to suppress the adiabatic heat.

The methods disclosed in JP-A-59-82101 and JP-A-58-25465, however, cannot produce a titanium alloy bar that simultaneously has excellent ductility, fatigue characteristics and formability.

Even if cross sectional area reduction rate per rolling is 40% or less according to the method of JP-A-59-82102, it is not sufficient to suppress the adiabatic heat for some kinds of titanium alloys. The method of JP-A-58-25465 also causes characteristics deterioration by hydrogen absorption caused by water cooling, and difficulty in accurate temperature control because of deformation resulted from rapid cooling.

The method described in Article 1 deals with a Ti-6Al-4V alloy. As described below, the method is not necessarily applicable to alloys which generate large adiabatic heat and therefor should be hot rolled in low temperature region, resulting in poor ductility, fatigue characteristics and formability.

FIG. 3 shows a relationship between temperature and rolling time during hot rolling for Ti-6Al-4V alloy and Ti-4.5AI-3V-2Fe-2Mo alloy.

The heating temperature was 950° C. for the Ti-6Al-4V alloy, and 850° C. for the Ti-4.5Al-3V-2Fe-2Mo alloy. The Ti-4.5Al-3V-2Fe-2Mo alloy has lower β transus than that of the Ti-6Al-4V alloy by 100° C. so that the heating temperature was reduced by the difference, thus selecting 850° C. as the heating temperature thereof. The rolling was conducted using a reverse rolling mill and tandem rolling mills, while selecting the same conditions of rolling speed, reduction rate and pass schedule to both alloys. The rolling speed of reverse rolling mill was 2.7 m/sec, and the rolling speed of tandem rolling mills was 2.25 m/sec at the final rolling pass where the rolling speed becomes the maximum for both alloys. The rolling speeds are lower than the rolling speed of Article 1 (6 m/sec). The cross sectional area reduction rate was selected to maximum 26% for both alloys.

For the case of the Ti-6Al-4V alloy, the rolling was conducted at a sufficiently lower temperature than 1000° C. which is the β transus of the alloy, thus giving favorable structure. For the case of the Ti-4.5Al-3V-2Fe-2Mo alloy, however, even if the heating temperature was decreased by the magnitude of low β transus, the low temperature rolling resulted in increased deformation resistance and in increased adiabatic heat, so the temperature increased to a temperature region exceeding the β transus, thus failed to obtain favorable microstructure. As a result, excellent ductility, fatigue characteristics and formability were not obtained. The result suggests that rolling conditions such as rolling temperature, reduction rate and time between rolling passes shall be considered, as well as the rolling speed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a high strength titanium alloy bar having excellent ductility, fatigue characteristics and formability, and to provide a method of manufacturing thereof.

The object is attained by an God type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar.

The α+β type titanium alloy bar can be manufactured by a method comprising the step of hot rolling an α+β type titanium alloy, consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, while keeping the surface temperature thereof to β transus or below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical method for manufacturing a bar.

FIG. 2A shows a process for hot rolling a bar using a reverse rolling mill.

FIG. 2B shows a process for hot rolling a bar using a tandem rolling mill.

FIG. 3 shows a relationship between temperature and rolling time during hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy.

FIG. 4 shows a relationship between average grain size of primary α phase and total elongation measured by high temperature tensile test.

FIG. 5 shows a relationship between average grain size of primary α phase and fatigue strength after 10⁶ cycles observed in fatigue test.

FIG. 6 shows temperature changes with time at the surface and the center.

FIG. 7 shows a relationship between cross sectional area and temperature difference between surface and center.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the present invention studied the microstructure of α+β type titanium alloy bar to provide excellent ductility, fatigue characteristics and formability, and found the followings.

The α+β type titanium alloy consists of primary α phase and transformed β phase. If, however, the alloy contains very large volume fraction of α phase that has HCP structure having little sliding system, or contains very large volume fraction of transformed β phase containing acicular α phase, formability and ductility deteriorate. Consequently, the volume fraction of primary α phase is specified to a range of from 10 to 90%. If the volume fraction of α phase and of β phase is equal or close to each other at reheating stage before hot roll ng, the formability becomes better, so the volume fraction of primary α phase is preferably between 50 and 80%.

FIG. 4 shows a relationship between average grain size of primary α phase and total elongation measured by high temperature tensile test.

When the average grain size of primary α phase exceeds 10 μm, the total elongation measured by high temperature tensile test rapidly decreases, and therefore the formability degrades.

FIG. 5 shows a relationship between average grain size of primary α phase and fatigue strength after 10⁸ cycles observed in fatigue test.

If the average grain size of primary α phase exceeds 10 μm, the fatigue strength decreases. If the average grain size of primary α phase becomes less than 6 μm, higher fatigue strength is attained.

Forging a bar induces rough surface on a free deforming plane not contacting with a mold due to the shape of grains, or due to the aspect ratio of the grains. Generally, the grains of bar tend to be elongated in the rolling direction. Particularly for the case of upset forging, elongated grains appear on a side face of the bar that becomes a free deforming plane. Therefore, it is necessary to avoid excessive increase in the aspect ratio during forging, more concretely to regulate the aspect ratio not exceeding 4 for the grains of the primary α phase on a cross section parallel in the rolling direction of the bar in order to prevent rough surface on the bar after forged.

Based on the above-described findings, a high strength titanium alloy bar having excellent ductility, fatigue characteristics and formability is obtained when the volume fraction of the primary α phase is between 10 and 90%, preferably between 50 and 80%, the average grain size in the primary α phase is 10 μm or less, preferably 6 μm or less, and further the aspect ratio of grains in the primary α phase is 4 or less.

The α+β type titanium alloy bar having above-described microstructure should consist essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti. The reasons to limit the content of individual elements are described below.

Al

Aluminum is an essential element to stabilize the α phase and to contribute to the strength increase. If the Al content is below 4%, high strength cannot fully be attained. If the Al content-exceeds 5%, ductility degrades.

V

Vanadium is an element to stabilize the β phase and to contribute to the strength increase. If the V content is below 2.5%, high strength cannot fully be attained, and β phase becomes unstable. If the V content exceeds 3.5%, range of workable temperature becomes narrow caused by the lowered β transus, and cost increases.

Mo

Molybdenum is an element to stabilize the β phase and to contribute to the strength increase. If the Mo content is below 1.5%, high strength cannot fully be attained, and β phase becomes unstable. If the Mo content exceeds 2.5%, range of workable temperature becomes narrow caused by the lowered β transus, and cost increases.

Fe

Iron is an element to stabilize the β phase and to contribute to the strength increase. Iron rapidly diffuses to improve formability. If, however, the Fe content is below 1.5%, high strength cannot fully be attained, and the β phase becomes unstable, which results in failing to attain excellent formability. If the Fe content exceeds 2.5%, range of workable temperature becomes narrow caused by the lowered β transus, and degradation in characteristics is induced by segregation.

The α+β type titanium alloy bar according to the present invention may be manufactured by hot rolling an α+β type titanium alloy having above-described composition while adjusting the conditions of heating temperature, rolling temperature range, reduction rate, rolling speed, time between passes, and other variables to suppress the temperature rise caused by the adiabatic g heat, namely to keep the surface temperature of the alloy not exceeding the β transus. For example, the method comprises the steps of: heating an α+β type titanium alloy having β transus of Tβ ° C. so that the surface temperature ranges between (Tβ-150) and Tβ ° C.; and hot rolling the heated α+β type titanium alloy so that the surface temperature thereof during hot rolling is between (Tβ-300) and (Tβ-50)° C., and so that the finish surface temperature thereof is between (Tβ-300) and (Tβ-100)° C.

The reason of heating the surface before hot rolling in the range of from (Tβ-150) to Tβ ° C. is the following. If the surface temperature before hot rolling is below (Tβ-150)° C., the decrease in temperature during the final rolling stage becomes significant to increase crack susceptibility and deformation resistance. And, if the surface temperature before hot rolling exceeds Tβ ° C., the microstructure of the bar becomes B microstructure consisting mainly of acicular α phase, which deteriorates ductility and formability. The reason of limiting the surface temperature during hot rolling to the range of from (Tβ-300) to (Tβ-50)° C. is the following. If the surface temperature during hot rolling is below (Tβ-300)° C., the hot formability deteriorates to induce problems such as cracking. And, if the surface temperature during hot rolling exceeds (Tβ-50)° C., the temperature rise caused by the adiabatic heat induces coarse grains and formation of acicular phase. The reason of limiting the finish surface temperature immediately after the final rolling pass to the range of from (Tβ-300) and (Tβ-100)° C. is the following. If the finish temperature thereof is below (Tβ-300)° C., the crack susceptibility and the deformation resistance increase. And, if the finish temperature thereof exceeds (Tβ-100)° C., grains become coarse.

The hot rolling is conducted by plurality of rolling passes. To prevent temperature rise caused by the adiabatic heat, it is preferable to keep the reduction rate not more than 40% per rolling pass.

When the hot rolling is conducted by a reverse rolling mill, it is preferable to limit the rolling speed not more than 6 m/sec to prevent the temperature rise caused by the adiabatic heat. When the hot rolling is conducted by tandem rolling mills, it is preferable to limit the rolling speed not more than 1.5 m/sec.

Since the alloy is cooled from surface after each rolling pass, the surface of the alloy receives temperature drop to some extent before entering succeeding pass even if a temperature rise exists caused by the adiabatic heat. As shown in FIG. 6, however, if the alloy has a large diameter (for the case of 106 mm in diameter), the temperature drop at center section of the alloy is small so that a large temperature difference appears between the surface and the center of the alloy. When the temperature drop at the center is small, the alloy is subjected to succeeding rolling pass before lowering the temperature of the center, which further increases the temperature owing to the adiabatic heat. If the phenomenon sustains, the center is hot rolled at higher temperature than the initial temperature. Consequently, the center of alloy having large diameter is required to be cooled with sufficient time between rolling passes.

To this point, the inventors of the present invention made a detailed study on the temperature difference between the surface and the center, and derived the finding described below. As shown in FIG. 7, the temperature difference significantly increases at or above 3500 mm² of cross sectional area of alloy normal to the rolling direction thereof. When an alloy having large cross sectional area is hot rolled to S mm² of the cross sectional area, securing the time before entering succeeding rolling at 0.167×S^(1/2) sec or more can make the temperature difference small and is favorable in manufacturing a bar having homogeneous characteristics.

According to the manufacturing method of the present invention, the hot rolling is carried out while keeping the surface temperature of the alloy to β transus or below, thus there is a possibility for the surface temperature to decrease to a lower than the required rolling temperature range during hot rolling depending on the time between rolling passes and on the diameter of alloy. In that case, reheating the alloy may be given using a high frequency heating unit or the like.

EXAMPLE 1

Materials having 125 square mm size were prepared by cutting each of the base alloy A01 (having composition within the range of the present invention) and the base alloy A02 (having composition outside the range of the present invention), both of which are α+β type titanium alloy having respective chemical compositions given in Table 1. The materials are hot rolled using a caliber rolling mill under respective conditions (B01 through B18) given in Table 2 to produce bars having 20 mm and 50 mm in diameter, respectively. For the time between rolling passes given in Table 2, ◯ denotes the time between rolling passes of 0.167×S^(1/2) or more for all the rolling passes under each rolling condition, and X denotes the time between rolling passes of less than 0.167×S^(1/2). Table 3 through Table 20 give cross sectional area S of alloy, reduction rate, 0.167×S^(1/2), time between rolling passes, surface temperature, and rolling speed on each rolling pass under each rolling condition. R in the table signifies a reverse rolling mill, and T signifies tandem rolling mills.

The produced bars were annealed at temperatures between 700 and 720° C. Tensile test was conducted to determine yield strength (0.2% PS), tensile strength (UTS), elongation (El), and reduction of area (RA). In addition, the smooth fatigue test (under the condition of Kt=1) and the notch fatigue test (under the condition of Kt=3) were given to determine fatigue strength.

Furthermore, optical microstructure examination was performed at the center of the bar and at the position of quarter of diameter (¼ D) to determine grain size of primary α phase, volume fraction of the grains, and aspect ratio of the grains on a cross section parallel in the rolling direction.

The results are given in Table 21. The columns of the microstructure in the table giving no grain size mean that the position consisted only of β microstructure consisting mainly of acicular α phase and that the equiaxed primary α phase could not be observed.

When the surface heating temperature is below (Tβ-150)° C., the surface temperature of the alloy was excessively low, and the rolling load became excessive to fail in rolling. When the heating temperature exceeds Tβ ° C., the surface temperature of the alloy became too high even if the time between rolling passes was within the range of the present invention, which is seen under the rolling conditions of B02 and B11, so the surface temperature exceeded Tβ ° C. caused by the adiabatic heat to form β microstructure consisting mainly of acicular α phase at the center of the bar, thus deteriorated ductility and fatigue characteristics.

When the finish surface temperature was below (Tβ-300)° C., the temperature of the alloy became too low, which deteriorated formability to generate cracks during hot rolling. When the finish surface temperature exceeded (Tβ-100)° C., fine microstructure could not be attained, deteriorating ductility and fatigue characteristics as in the cases under the conditions of B04, B05, and B07.

When the surface temperature during hot rolling was below (Tβ-300)° C., the surface temperature was too low, generating cracks. When the surface temperature exceeded (Tβ-50)° C., the center and the ¼ D had β microstructure consisting mainly of acicular α phase after hot rolling, deteriorating ductility and fatigue characteristics.

When the reduction rate per rolling pass exceeded 40%, the adiabatic heat was enhanced, and the temperature of the alloy exceeded Tβ ° C., and fine microstructure could not be attained.

In the case of the rolling condition B14 which applied a reverse rolling mill and which selected the rolling speeds of higher than 6 m/sec, or in the case of rolling condition B15 which applied tandem rolling mills and which selected the rolling speeds of higher than 1.5 m/sec; the adiabatic heat became large, and the surface temperature exceeded Tβ ° C., thus failed to attain fine microstructure.

When the time between rolling passes was outside the range of the present invention, the surface temperature increase caused by the adiabatic heat overrode the temperature decrease caused by air cooling, thus the surface temperature exceeded Tβ ° C., and fine microstructure could not be attained.

With the bars using A01 which had the chemical composition within the range of the present invention and produced under the rolling conditions B01, B06, B08, B09, B16, B17, and B18, homogeneous microstructure of 10 μm or smaller grain size of primary α phase was observed, and they provided excellent ductility and fatigue characteristics. That is, further excellent ductility and fatigue characteristics could be attained giving 15% or larger elongation, 40% or larger reduction of area, 500 MPa or larger smooth fatigue strength, and 200 MPa of notch (Kt=3) fatigue strength. Furthermore, with the α+β type titanium alloy bars having 50 to 80% of volume fraction of primary α phase and 6 μm or less of average grain size of primary α phase, produced under the rolling conditions of B01, B06, B08, and B09, further excellent ductility and fatigue characteristics could be attained giving 20% or larger elongation, 50% or larger reduction of area, 550 MPa or larger smooth fatigue strength, and 200 MPa of notch (Kt=3) fatigue strength.

On the other hand, bars produced using A02 having chemical composition outside the range of the present invention under the rolling conditions of B10 and B12 could not attain satisfactory ductility and fatigue characteristics because the grain size in the primary α phase exceeded 10 μm, though the adiabatic heat was suppressed because the rolling conditions were within the range of the present invention.

EXAMPLE 2

Cylindrical specimens having 8 mm in diameter and 12 mm in height were cut from the center section in radial direction of bars produced in Example 1 under the rolling conditions B01 through B18, respectively. The specimens were heated to 800° C. and were compressed to 70%. After the compression, the occurrence of cracks and of rough surface on the surface of each specimen was inspected to give evaluation of hot forging property.

The results are shown in Table 21.

As for the bars produced under the rolling conditions of B01, B06, B08, B09, B16, B17, and B18 which were within the range of the present invention, no crack and rough surface appeared, and favorable hot forging property was obtained.

On the other hand, for the bars produced under the rolling conditions of B10 and B12 in which the grain size in the primary α phase exceeded 10 μm, rough surface appeared, though no crack was generated. As for the bars having only α phase at center and ¼ D produced under the rolling conditions of B02, B03, B04, B05, B07, B11, B14, and B15, both cracks and rough surface appeared. Furthermore, for the bars produced under the rolling condition B14 giving aspect ratios of more than 4 for the grains in a cross section parallel in the rolling direction, though giving the grain size in the primary α phase and the volume fraction within the range of the present invention, rough surface also appeared. TABLE 1 β Alloy Al V Fe Mo O C N H transus A01 4.7 3.1 2.1 1.9 0.1 0.001 0.005 0.0017  900 °C. A02 6.1 4.1 0.2 — 0.2 0.01 0.006 0.0016 1000 °C. Unit is mass %.

TABLE 2 Rolling speed Final rolling Maximum in speed in Rolling Total reduction rough rolling finish rolling Finish Reheating temp. Finish Time number rate (Reverse (Tandem Rolling diameter temp. range temp. between of per rolling rolling mill) rolling mills) condition Alloy (mm) (° C.) (° C.) (° C.) passes passes pass (%) (m/sec) (m/sec) Remark B01 A01 φ20 800 700-811 714 ∘ 17 25.8 2.7 1.125 E B02 A01 φ20 950 755-929 765 ∘ 17 25.8 2.7 1.125 C B03 A01 φ20 890 754-911 764 ∘ 17 25.8 2.7 1.125 C B04 A01 φ20 850 818-930 919 ∘ 8 42.4 2.7 1.125 C B05 A01 φ20 800 845-901 865 x 17 25.8 2.7 1.125 C B06 A01 φ50 800 711-804 731 ∘ 12 18.4 2.7 1.125 E B07 A01 φ50 830 864-909 874 x 12 18.4 2.7 1.125 C B08 A01 φ20 800 670-812 690 ∘ 17 25.8 2.7 1.125 E B09 A01 φ20 820 721-829 726 ∘ 17 25.8 2.7 1.125 E B10 A02 φ20 900 791-887 806 ∘ 17 25.8 2.7 1.125 C B11 A02 φ20 1050   815-1024 825 ∘ 17 25.8 2.7 1.125 C B12 A02 φ50 900 810-906 830 ∘ 12 18.4 2.7 1.125 C B13 A01 φ20 920 698-928 698 ∘ 17 25.8 2.7 1.125 C B14 A01 φ20 800 774-911 774 ∘ 17 25.8 10.8  1.125 C B15 A01 φ20 800 719-910 864 ∘ 17 25.8 2.7 2.250 C B16 A01 φ50 830 764-845 766 ∘ 12 18.4 2.7 1.125 E B17 A01 φ20 830 757-842 777 ∘ 17 25.8 2.7 1.125 E B18 A01 φ20 865 772-850 772 ∘ 17 25.8 2.7 1.125 E E: Example, C: Comparative example Numerals with underline signify that they are outside the range of the present invention.

TABLE 3 Rolling condition: B01 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 790 R 2 11000 15.4 17.5 25 2.7 796 R 3 9500 13.6 16.3 25 2.7 801 R 4 8000 15.8 14.9 25 2.7 803 R 5 6500 18.8 13.5 25 2.7 811 R 6 5200 20.0 12.0 25 2.7 801 R 7 4150 20.2 10.8 25 2.7 779 R 8 3300 20.5 9.6 25 2.7 761 R 9 2450 25.8 8.3 25 2.7 738 R 10 1850 24.5 7.2 25 2.7 719 R 11 1450 21.6 6.4 5 0.350 721 T 12 1150 20.7 5.7 5 0.466 732 T 13 900 21.7 5.0 5 0.581 739 T 14 700 22.2 4.4 5 0.733 745 T 15 550 21.4 3.9 5 0.871 741 T 16 420 23.6 3.4 5 0.982 730 T 17 320 23.8 1.125 714 T

TABLE 4 Rolling condition: B02 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 929 R 2 11000 15.4 17.5 25 2.7 925 R 3 9500 13.6 16.3 25 2.7 919 R 4 8000 15.8 14.9 25 2.7 913 R 5 6500 18.8 13.5 25 2.7 911 R 6 5200 20.0 12.0 25 2.7 900 R 7 4150 20.2 10.8 25 2.7 891 R 8 3300 20.5 9.6 25 2.7 880 R 9 2450 25.8 8.3 25 2.7 868 R 10 1850 24.5 7.2 25 2.7 860 R 11 1450 21.6 6.4 5 0.350 852 T 12 1150 20.7 5.7 5 0.466 839 T 13 900 21.7 5.0 5 0.581 829 T 14 700 22.2 4.4 5 0.733 822 T 15 550 21.4 3.9 5 0.871 803 T 16 420 23.6 3.4 5 0.982 785 T 17 320 23.8 1.125 765 T

TABLE 5 Rolling condition: B03 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 890 R 2 11000 15.4 17.5 25 2.7 894 R 3 9500 13.6 16.3 25 2.7 899 R 4 8000 15.8 14.9 25 2.7 906 R 5 6500 18.8 13.5 25 2.7 911 R 6 5200 20.0 12.0 25 2.7 902 R 7 4150 20.2 10.8 25 2.7 889 R 8 3300 20.5 9.6 25 2.7 881 R 9 2450 25.8 8.3 25 2.7 867 R 10 1850 24.5 7.2 25 2.7 860 R 11 1450 21.6 6.4 5 0.350 852 T 12 1150 20.7 5.7 5 0.466 839 T 13 900 21.7 5.0 5 0.581 830 T 14 700 22.2 4.4 5 0.733 820 T 15 550 21.4 3.9 5 0.871 803 T 16 420 23.6 3.4 5 0.982 784 T 17 320 23.8 1.125 764 T

TABLE 6 Rolling condition: B04 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 9300 40.5 19.0 25 2.7 849 R 2 5500 40.9 17.5 25 2.7 865 R 3 3300 40.0 16.3 25 2.7 879 R 4 1900 42.4 14.9 25 2.7 896 R 5 1100 42.1 13.5 25 2.7 912 R 6 660 40.0 12.0 25 2.7 921 R 7 400 39.4 10.8 25 2.7 930 R 8 320 20.0 2.7 919 R

TABLE 7 Rolling condition: B05 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 10 2.7 791 R 2 11000 15.4 17.5 10 2.7 805 R 3 9500 13.6 16.3 10 2.7 819 R 4 8000 15.8 14.9 10 2.7 836 R 5 6500 18.8 13.5 10 2.7 850 R 6 5200 20.0 12.0 10 2.7 865 R 7 4150 20.2 10.8 10 2.7 871 R 8 3300 20.5 9.6 10 2.7 875 R 9 2450 25.8 8.3 10 2.7 879 R 10 1850 24.5 7.2 10 2.7 884 R 11 1450 21.6 6.4 5 0.350 901 T 12 1150 20.7 5.7 5 0.466 899 T 13 900 21.7 5.0 5 0.581 895 T 14 700 22.2 4.4 5 0.733 895 T 15 550 21.4 3.9 5 0.871 883 T 16 420 23.6 3.4 5 0.982 875 T 17 320 23.8 1.125 860 T

TABLE 8 Rolling condition: B06 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 791 R 2 11000 15.4 17.5 25 2.7 796 R 3 9500 13.6 16.3 25 2.7 801 R 4 8000 15.8 14.9 25 2.7 804 R 5 6700 16.3 13.7 25 2.7 806 R 6 6000 10.5 12.9 25 2.7 784 R 7 5200 13.3 12.0 25 2.7 764 R 8 4650 10.6 11.4 25 2.7 746 R 9 3800 18.3 10.3 25 2.7 733 R 10 3100 18.4 9.3 5 0.622 733 T 11 2600 16.1 8.5 5 0.837 734 T 12 2210 15.0 1.125 731 T

TABLE 9 Rolling condition: B07 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 10 2.7 819 R 2 11000 15.4 17.5 10 2.7 836 R 3 9500 13.6 16.3 10 2.7 849 R 4 8000 15.8 14.9 10 2.7 873 R 5 6700 16.3 13.5 10 2.7 879 R 6 6000 10.5 12.9 10 2.7 896 R 7 5200 13.3 12.0 10 2.7 901 R 8 4650 10.6 11.4 10 2.7 904 R 9 3800 18.3 10.3 5 2.7 909 R 10 3100 18.4 9.3 5 0.622 902 T 11 2600 16.1 8.5 5 0.837 883 T 12 2210 15.0 1.125 874 T

TABLE 10 Rolling condition: B08 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 790 R 2 11000 15.4 17.5 25 2.7 795 R 3 9500 13.6 16.3 25 2.7 799 R 4 8000 15.8 14.9 25 2.7 804 R 5 6500 18.8 13.5 25 2.7 812 R 6 5200 20.0 12.0 25 2.7 800 R 7 4150 20.2 10.8 25 2.7 780 R 8 3300 20.5 9.6 25 2.7 759 R 9 2450 25.8 8.3 25 2.7 741 R 10 1850 24.5 7.2 25 2.7 720 R 11 1450 21.6 6.4 10 0.350 719 T 12 1150 20.7 5.7 10 0.466 724 T 13 900 21.7 5.0 10 0.581 730 T 14 700 22.2 4.4 10 0.733 729 T 15 550 21.4 3.9 10 0.871 721 T 16 420 23.6 3.4 10 0.982 705 T 17 320 23.8 1.125 690 T

TABLE 11 Rolling condition: B09 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 810 R 2 11000 15.4 17.5 25 2.7 816 R 3 9500 13.6 16.3 25 2.7 821 R 4 8000 15.8 14.9 25 2.7 824 R 5 6500 18.8 13.5 25 2.7 829 R 6 5200 20.0 12.0 25 2.7 821 R 7 4150 20.2 10.8 25 2.7 800 R 8 3300 20.5 9.6 25 2.7 779 R 9 2450 25.8 8.3 25 2.7 761 R 10 1850 24.5 7.2 25 2.7 749 R 11 1450 21.6 6.4 5 0.350 741 T 12 1150 20.7 5.7 5 0.466 751 T 13 900 21.7 5.0 5 0.581 760 T 14 700 22.2 4.4 5 0.733 766 T 15 550 21.4 3.9 5 0.871 761 T 16 420 23.6 3.4 5 0.982 751 T 17 320 23.8 1.125 726 T

TABLE 12 Rolling condition: B10 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 886 R 2 11000 15.4 17.5 25 2.7 884 R 3 9500 13.6 16.3 25 2.7 884 R 4 8000 15.8 14.9 25 2.7 887 R 5 6500 18.8 13.5 25 2.7 885 R 6 5200 20.0 12.0 25 2.7 859 R 7 4150 20.2 10.8 25 2.7 841 R 8 3300 20.5 9.6 25 2.7 820 R 9 2450 25.8 8.3 25 2.7 800 R 10 1850 24.5 7.2 25 2.7 791 R 11 1450 21.6 6.4 5 0.350 801 T 12 1150 20.7 5.7 5 0.466 810 T 13 900 21.7 5.0 5 0.581 830 T 14 700 22.2 4.4 5 0.733 836 T 15 550 21.4 3.9 5 0.871 829 T 16 420 23.6 3.4 5 0.982 821 T 17 320 23.8 1.125 806 T

TABLE 13 Rolling condition: B11 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 1024 R 2 11000 15.4 17.5 25 2.7 1015 R 3 9500 13.6 16.3 25 2.7 1003 R 4 8000 15.8 14.9 25 2.7 996 R 5 6500 18.8 13.5 25 2.7 985 R 6 5200 20.0 12.0 25 2.7 969 R 7 4150 20.2 10.8 25 2.7 961 R 8 3300 20.5 9.6 25 2.7 949 R 9 2450 25.8 8.3 25 2.7 930 R 10 1850 24.5 7.2 25 2.7 921 R 11 1450 21.6 6.4 5 0.350 911 T 12 1150 20.7 5.7 5 0.466 901 T 13 900 21.7 5.0 5 0.581 891 T 14 700 22.2 4.4 5 0.733 881 T 15 550 21.4 3.9 5 0.871 864 T 16 420 23.6 3.4 5 0.982 845 T 17 320 23.8 1.125 825 T

TABLE 14 Rolling condition: B12 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 891 R 2 11000 15.4 17.5 25 2.7 895 R 3 9500 13.6 16.3 25 2.7 899 R 4 8000 15.8 14.9 25 2.7 905 R 5 6700 16.3 13.7 25 2.7 906 R 6 6000 10.5 12.9 25 2.7 886 R 7 5200 13.3 12.0 25 2.7 865 R 8 4650 10.6 11.4 25 2.7 845 R 9 3800 18.3 10.3 25 2.7 836 R 10 3100 18.4 9.3 5 0.622 835 T 11 2600 16.1 8.5 5 0.837 834 T 12 2210 15.0 1.125 830 T

TABLE 15 Rolling condition: B13 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 929 R 2 11000 15.4 17.5 25 2.7 925 R 3 9500 13.6 16.3 25 2.7 919 R 4 8000 15.8 14.9 25 2.7 913 R 5 6500 18.8 13.5 25 2.7 911 R 6 5200 20.0 12.0 25 2.7 900 R 7 4150 20.2 10.8 25 2.7 891 R 8 3300 20.5 9.6 25 2.7 880 R 9 2450 25.8 8.3 25 2.7 868 R 10 1850 24.5 7.2 25 2.7 850 R 11 1450 21.6 6.4 10 0.350 832 T 12 1150 20.7 5.7 10 0.466 804 T 13 900 21.7 5.0 10 0.581 777 T 14 700 22.2 4.4 10 0.733 749 T 15 550 21.4 3.9 10 0.871 728 T 16 420 23.6 3.4 10 0.982 713 T 17 320 23.8 1.125 698 T

TABLE 16 Rolling condition: B14 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 10.8 810 R 2 11000 15.4 17.5 25 10.8 836 R 3 9500 13.6 16.3 25 10.8 861 R 4 8000 15.8 14.9 25 10.8 883 R 5 6500 18.8 13.5 25 10.8 911 R 6 5200 20.0 12.0 25 10.8 901 R 7 4150 20.2 10.8 25 10.8 869 R 8 3300 20.5 9.6 25 1.8 841 R 9 2450 25.8 8.3 25 10.8 808 R 10 1850 24.5 7.2 25 10.8 779 R 11 1450 21.6 6.4 10 0.350 781 T 12 1150 20.7 5.7 10 0.466 792 T 13 900 21.7 5.0 10 0.581 799 T 14 700 22.2 4.4 10 0.733 805 T 15 550 21.4 3.9 10 0.871 801 T 16 420 23.6 3.4 10 0.982 790 T 17 320 23.8 1.125 774 T

TABLE 17 Rolling condition: B15 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 790 R 2 11000 15.4 17.5 25 2.7 796 R 3 9500 13.6 16.3 25 2.7 801 R 4 8000 15.8 14.9 25 2.7 803 R 5 6500 18.8 13.5 25 2.7 811 R 6 5200 20.0 12.0 25 2.7 801 R 7 4150 20.2 10.8 25 2.7 779 R 8 3300 20.5 9.6 25 2.7 761 R 9 2450 25.8 8.3 25 2.7 738 R 10 1850 24.5 7.2 25 2.7 719 R 11 1450 21.6 6.4 5 0.700 751 T 12 1150 20.7 5.7 5 0.932 782 T 13 900 21.7 5.0 5 1.162 829 T 14 700 22.2 4.4 5 1.466 865 T 15 550 21.4 3.9 5 1.742 891 T 16 420 23.6 3.4 5 1.964 910 T 17 320 23.8 2.500 864 T

TABLE 18 Rolling condition: B16 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 821 R 2 11000 15.4 17.5 25 2.7 817 R 3 9500 13.6 16.3 25 2.7 834 R 4 8000 15.8 14.9 25 2.7 838 R 5 6700 16.3 13.7 25 2.7 845 R 6 6000 10.5 12.9 25 2.7 824 R 7 5200 13.3 12.0 25 2.7 794 R 8 4650 10.6 11.4 25 2.7 776 R 9 3800 18.3 10.3 25 2.7 767 R 10 3100 18.4 9.3 5 0.622 764 T 11 2600 16.1 8.5 5 0.837 769 T 12 2210 15.0 1.125 766 T

TABLE 19 Rolling condition: B17 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 822 R 2 11000 15.4 17.5 25 2.7 825 R 3 9500 13.6 16.3 25 2.7 833 R 4 8000 15.8 14.9 25 2.7 834 R 5 6500 18.8 13.5 25 2.7 842 R 6 5200 20.0 12.0 25 2.7 830 R 7 4150 20.2 10.8 25 2.7 809 R 8 3300 20.5 9.6 25 2.7 790 R 9 2450 25.8 8.3 25 2.7 765 R 10 1850 24.5 7.2 25 2.7 757 R 11 1450 21.6 6.4 5 0.350 759 T 12 1150 20.7 5.7 5 0.466 772 T 13 900 21.7 5.0 5 0.581 771 T 14 700 22.2 4.4 5 0.733 774 T 15 550 21.4 3.9 5 0.871 771 T 16 420 23.6 3.4 5 0.982 779 T 17 320 23.8 1.125 777 T

TABLE 20 Rolling condition: B18 Number of Cross sectional Reduction 0.167{square root}S Time between Rolling speed Temp. Rolling passes area (mm²) rate (%) (sec) passes (sec) (m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 850 R 2 11000 15.4 17.5 25 2.7 847 R 3 9500 13.6 16.3 25 2.7 847 R 4 8000 15.8 14.9 25 2.7 845 R 5 6500 18.8 13.5 25 2.7 844 R 6 5200 20.0 12.0 25 2.7 845 R 7 4150 20.2 10.8 25 2.7 843 R 8 3300 20.5 9.6 25 2.7 834 R 9 2450 25.8 8.3 25 2.7 830 R 10 1850 24.5 7.2 25 2.7 829 R 11 1450 21.6 6.4 5 0.350 821 T 12 1150 20.7 5.7 5 0.466 814 T 13 900 21.7 5.0 5 0.581 803 T 14 700 22.2 4.4 5 0.733 794 T 15 550 21.4 3.9 5 0.871 790 T 16 420 23.6 3.4 5 0.982 782 T 17 320 23.8 1.125 772 T

TABLE 21 Microstructure (primary α) Forging Fatigue ¼ D characteristics strength Volume Center section Occur- Rolling 0.2% Smooth Notch Grain frac- Grain Volume Occur- rence con- PS UTS E1 RA test test size tion Aspect size fraction Aspect rence of rough Re- dition (MPa) (MPa) (%) (%) (Kt = 1) (Kt = 3) (μm) (%) ratio (μm) (%) ratio of crack surface mark B01 931 1030 20.4 51.9 565 230 2.5 66 1.5 2.7 66 1.8 Not Not E occurred occurred B02 885 1009 3.5 12.3 350 120 3.7 59 4.1 — — — Occurred Occurred C B03 879 1010 4.1 13.5 355 125 3.4 58 4.4 — — — Occurred Occurred C B04 881 1011 4.1 11.6 365 115 — — — — — — Occurred Occurred C B05 874 1014 3.8 11.1 360 100 3.8 29 4.2 — — — Occurred Occurred C B06 921 1020 20.0 50.8 560 225 5.4 60 2.1 5.8 68 2.2 Not Not E occurred occurred B07 887 1005 3.7 12.1 355 120 5.9 31 4.3 — — — Occurred Occurred C B08 930 1030 20.5 52.3 570 240 1.7 67 1.9 1.9 69 2.3 Not Not E occurred occurred B09 929 1027 20.1 50.1 550 210 4.1 62 1.7 4.9 64 2.1 Not Not E occurred occurred B10 911 1019 14.8 43.3 480 185 11.4 89 2.8 12.0 88 3.2 Not Occurred C occurred B11 863 1012 3.6 9.8 230 95 13.2 85 2.9 — — — Occurred Occurred C B12 902 1011 13.8 42.1 440 175 14.5 80 3.0 15.0 89 3.4 Not Occurred C occurred B13 899 987 12.1 38.2 395 155 5.5 85 4.2 5.8 87 4.5 Not Occurred C occurred B14 884 971 13.7 34.5 345 115 5.2 84 4.2 — — — Occurred Occurred C B15 894 955 11.9 33.3 340 120 5.3 81 4.3 — — — Occurred Occurred C B16 910 1014 17.4 40.1 505 205 6.2 63 2.5 6.4 60 2.7 Not Not E occurred occurred B17 914 1021 18.3 42.3 510 205 5.8 64 2.7 6.3 61 2.9 Not Not E occurred occurred B18 902 1008 15.6 40.1 500 200 6.5 60 3.1 6.6 60 3.3 Not Not E occurred occurred E: Example, C: Comparative example 

1. A method for manufacturing an α+β titanium alloy bar comprising hot rolling an α+β titanium alloy consisting essentially of 4 to 5 mass % Al, 2.5 to 3.5 mass % V, 1.5 to 2.5 mass % Fe, 1.5 to 2.5 mass % Mo and a balance of Ti, while keeping the surface temperature thereof to a temperature of f transus or below.
 2. The method for manufacturing an α+β titanium alloy bar of claim 1, wherein the α+β titanium alloy is hot rolled at a reduction rate of 40% or less per rolling pass.
 3. The method for manufacturing an α+β titanium alloy bar of claim 1, wherein the hot rolling is carried out at a rolling speed of 6 m/sec or less with a reverse rolling mill.
 4. The method for manufacturing an α+β titanium alloy bar of claim 1, wherein the hot rolling is carried out at a rolling speed of 1.5 m/sec or less with tandem rolling mills.
 5. The method for manufacturing an α+β titanium alloy bar of claim 1, wherein a waiting time before starting a succeeding rolling is 0.167×S^(1/2) or more seconds when the α+β titanium alloy has a cross sectional area of 3500 mm² or more in a direction normal to the rolling direction is hot rolled to a cross sectional area of S mm².
 6. The method for manufacturing an α+β titanium alloy bar of claim 1, wherein the α+β titanium alloy is reheated during the hot rolling.
 7. The method for manufacturing an α+β titanium alloy bar of claim 2, wherein the hot rolling is carried out at a rolling speed of 6 m/sec or less with a reverse rolling mill.
 8. The method for manufacturing an α+β titanium alloy bar of claim 2, wherein the hot rolling is carried out at a rolling speed of 1.5 m/sec or less with tandem rolling mills.
 9. The method for manufacturing an α+β titanium alloy bar of claim 2, wherein the α+β titanium alloy is reheated during hot rolling.
 10. A method for manufacturing an α+β titanium alloy bar comprising heating an α+β titanium alloy having a β transus temperature of Tβ° C. and consisting essentially of 4 to 5 mass % Al, 2.5 to 3.5 mass % V, 1.5 to 2.5 mass % Fe, 1.5 to 2.5 mass % Mo and a balance of Ti, so that the surface temperature thereof is between (Tβ-150) and Tβ° C. to provide a heated α+β titanium alloy; and hot rolling the heated α+β titanium alloy while keeping the surface temperature thereof during the hot rolling between (Tβ-300) and (Tβ-50)° C. and keeping a finish surface temperature thereof, as a surface temperature immediately after a final rolling pass, between (Tβ-300) and (Tβ-100)° C.
 11. The method for manufacturing an α+β titanium alloy bar of claim 10, wherein the α+β titanium alloy is hot rolled at a reduction rate of 40% or less per rolling pass.
 12. The method for manufacturing an α+β titanium alloy bar of claim 10, wherein the hot rolling is carried out at a rolling speed of 6 m/sec or less with a reverse rolling mill.
 13. The method for manufacturing an α+β titanium alloy bar of claim 10, wherein the hot rolling is carried out at a rolling speed of 1.5 m/sec or less with tandem rolling mills.
 14. The method for manufacturing an α+β titanium alloy bar of claim 10, wherein a waiting time before starting a succeeding rolling is 0.167×S^(1/2) or more seconds when a cross sectional area of 3500 mm² or more in a direction normal to the rolling direction is hot rolled to a cross sectional area of S mm².
 15. The method for manufacturing an α+β titanium alloy bar of claim 10, wherein the α+β titanium alloy is reheated during the hot rolling.
 16. The method for manufacturing an α+β titanium alloy bar of claim 11, wherein the hot rolling is carried out at a rolling speed of 6 m/sec or less with a reverse rolling mill.
 17. The method for manufacturing an α+β titanium alloy bar of claim 11, wherein the hot rolling is carried out at a rolling speed of 1.5 m/sec or less with tandem rolling mills.
 18. The method for manufacturing an α+β titanium alloy bar of claim 11, wherein the α+β titanium alloy is reheated during hot rolling.
 19. The method for manufacturing an α+β titanium alloy bar of claim 16, wherein the α+β titanium alloy is reheated during hot rolling.
 20. The method for manufacturing an α+β titanium alloy bar of claim 17, wherein the α+β titanium alloy is reheated during hot rolling. 